(USA Today) -- It may be the season for blue skies and sunshine, but the color scheme for books this summer has one shade: grey.

British writer E.L. James' erotic trilogy Fifty Shades of Grey, Fifty Shades Darker and Fifty Shades Freed has surged up like a publishing derecho. In bedrooms and book clubs, on mommy blogs and best-seller lists, it's all about the blindfold, the billionaire and the "red room of pain." In the USA, 20 million copies have been sold, and the 1,594-page trilogy has held the top three spots on USA TODAY's Best-Selling Books list for nine weeks.

Why are millions of readers, most of them women, devouring the trilogy and praying the rumor that James is writing a fourth book is true? Here are 10 reasons Greyis the new green in book publishing.

1. Sex sells, but what women really crave is love

Gentlemen, don't get your hopes up that the women of America have developed an insatiable appetite for pornography and multiple partners.

Although the trilogy is often and insultingly called "mommy porn," James is actually writing romance, the most popular category in book publishing. Despite its scarlet reputation, the series is an old-fashioned love story with some odd sex toys, riding crops and mild bondage tossed in. It's the classic boy-meets-girl, boy-loses-girl, boy-gets-girl-back plot that has fueled an infinity of romance novels.

James' hero is a 27-year-old alpha male Seattle billionaire hottie named Christian Grey, who is hauling a freight train of emotional baggage from his abusive childhood.

The heroine is a 21-year-old just-hatched college grad from Washington State. Though smart and hardworking, innocent Anastasia Steele is an untouched virgin who knows far more about 19th-century English literature than about modern men or sex.

"That is a very old romance trope - the alpha male who literally sweeps the heroine off her feet and into his life," says Sarah Wendell, author of Everything I Know About Love I Learned From Romance Novels.

2. It's all about the handcuffs

The romance genre has many categories: historical, paranormal, Regency, inspirational, contemporary and, yes, erotic. At the moment, James is the most popular, but other writers are hot on her trail; Sylvia Day's erotic romance Bared to You is No. 9 on USA TODAY's list this week.

Back to the cuffs, you order?

Blame them on mommy.

Although adopted into a wealthy, loving family as a tot, Grey has deep psychological wounds from his childhood. He's not just a control freak, he's the MVP on the all-star roster of control freaks. He finds release in dominating submissive women in his playroom of perversion.

His bank account allows him to indulge this compulsion with comely young women. Because this is a family publication and not a guide to the joys of consensual bondage and spanking, let's just say he ties Ana up in physical and emotional knots while she discovers her powers as a sexual healer.

And while the earth moves incessantly for these two, the level of kink is pretty tame to anyone familiar with, say, 1954's The Story of O.

3. A publishing fairy tale

Penniless Ana's taming of the billionaire bad boy is gritty reality compared with the Cinderella story of how a fortysomething London TV producer named Erika Leonard went from fan-fiction creator to the global goddess of books known as "E.L. James."

Fifty Shades of Grey was published in 2011 by a small Australian publisher. Available as an e-book, it grew through word of mouth, becoming a finalist for Goodreads' 2011 romance of the year. After the e-book hit U.S. best-seller lists in early 2012, Vintage Books bought the rights in March to publish the e-book and paperback for seven figures. Universal Films/Focus Features has bought the film rights.

4. Welcome to the Twilight zone

Stephenie Meyer's, to be precise. James got her start writing fan fiction about Twilight's Bella and Edward Cullen under the pseudonym Snowqueen's Icedragon. She dropped the vampire blood-sucking stuff and added sex. Fifty Shades of Grey has its origins in James' online serialized story "Master of the Universe." Like Twilight, the Fifty Shadestrilogy is set in the Pacific Northwest and shares the same obsessive "is this love or do I need a restraining order?" vibe.

5. The writing

It's not just James' characters grappling with strong emotions. Readers are black or white about her talent. On Amazon, Fifty Shades is rated five stars by almost 3,500 customers, while more than 2,500 gave it one star. To her admirers, James is a genius who writes with unflinching honesty about forbidden sexual desires to possess and punish, to submit and succumb. They adore the complicated relationship between insecure Ana and bossy Christian and the way it changes over the course of the three books. No editor or publicist can manufacture this kind of addictive pull. J.K. Rowling had it with Harry Potter,Dan Brown had it with The Da Vinci Code, Suzanne Collins has it with The Hunger Games. As HarperCollins senior editor May Chen puts it, "magic happens."

James' critics include people who say the series is a Twilight ripoff. (Not Meyer, who told MTV: "I've heard about it; I haven't really gotten into it that much. Good on her - she's doing well. That's great!") Some find the sex stuff unhealthy and retro. The biggest complaint, however: James' breathless "Holy cow!" prose, with its endless lip biting and "inner goddess" references. In TheChicago Tribune, Jessica Reaves wrote, "This is a pretty dreadful book. Put simply, author E.L. James - who is now officially invulnerable to criticism because she has more money than God - is not a very good writer."

6. Lifestyle porn

Sexual tastes might differ, but doesn't everyone adore presents? Christian's gifts to Ana begin with first-edition copies of Thomas Hardy's 1891 Tess of the d'Urbervilles, then move on to fancy new cars, an entire company, a top-of-the-line Mac laptop not yet on the market, jewelry, closets of new designer clothes and $3,295 Christian Louboutinshoes. For many stressed-out readers, the trilogy's real love object is the housekeeper who whips up three squares a day when she's not mopping down the "red room of pain."

7. E-books are today's brown paper wrapper

With e-books, there's no cashier to judge you or your frisky bedtime reading. Online, it's just you and the buy button. Fifty Shadesof Grey made its debut on USA TODAY's best-seller list on Feb. 16, months before the Vintage paperback was in stores.

Atria Books publisher Judith Curr thinks that having the series first appear in e-book form helped propel it to the top. "People were able to indulge their curiosity anonymously," Curr says.

By the time the paperback arrived, it had become such a phenomenon, "the taboo had been removed."

8. An answer to Freud's query about what women want

Christian Grey, of course.

The 27-year-old self-made entrepreneur dropped out of Harvard (hello, Mark Zuckerberg). He is a smoking-hot Adonis (hello, Channing Tatum). He has his own security force to keep his family safe (hello, Michael Corleone). He's also an innovative philanthropist (hello, Bill Gates).

But those guys are so busy. Not Christian. This CEO spends his days like a smitten seventh-grader. He and Ana e-mail constantly. He enjoys washing, drying and braiding Ana's hair. He utters lines that few women have heard anywhere but in their dreams. "I could watch you sleep for hours, Anastasia."

He doesn't want a supermodel, a movie star or a high-achieving lady doctor (hello, the new Mrs. Zuckerberg). The only woman in the world for him is a pretty 21-year-old with a tendency to blush, who insists, "I am not a merger. I am not an acquisition."

9. Fifty shades of inescapable

In less than six months, James' trilogy has made the hyper-jump from the little pond of books to the global ocean of pop culture. Tina Brown put it on the cover of Newsweek, claiming "Why surrender is a feminist dream." At 84, Dr. Ruth is thumbs-up about Christian. Bret Easton Ellis wants to write the screenplay. It's been analyzed on CNN and parodied on Saturday Night Live.

10. Marital aid

The series' most vocal fans are married women who adore James' core message about the redemptive power of love. Wives are buying their husbands "Grey" ties, shopping for the sex toys featured in Fifty Shades, and blogging that the novels have reignited the pilot light of passion in their bedrooms.

Yup, an NC-17 trilogy is reinforcing the bedrock foundation of family values: marriage.

"I think this book took off because back in January and February, women were willing to go on the record in the news with their names and the towns they lived in saying: 'This was great for my sex life. This was great for my marriage,' " says Wendell, who is also a romance blogger. "Women's arousal is a topic that's greeted with shame, mocking or both. ... Women were frank about the fact that this book was arousing, and they told other women."

So before you dismiss Fifty Shades as silly or demeaning having never read a word, here's Christian Grey's advice: "We are consenting adults and what we do behind closed doors is between ourselves. You need to free your mind and listen to your body."